1. Field of the Invention
The present invention relates to a curved-section-information supplying apparatus and a curved-section-information supplying method that supply curved-section information representing the shape of a clothoid section included in a curved section of a road.
2. Description of the Related Art
As an example, on an expressway R like that illustrated in FIG. 1, there is a road section that continues from a straight section SsIN to a curved section Sc and further continues from the curved section Sc to a straight section SsOUT. The curved section Sc in such a road section is, in many cases, designed so as to have a shape obtained by connecting a clothoid section SclIN having a shape that follows a clothoid curve, an arc section So having a shape that follows an arc curve, and another clothoid section SclOUT, as schematically illustrated in FIGS. 1 and 2. As illustrated in FIG. 3, a relationship between a curve length L from a start point Pst of a clothoid curve CVcl and a radius of curvature, R, at the position of the curve length L is represented by:L·R=A2.
In this equation, because the curve length L and the radius of curvature, R, at the position of the curve length L are in inverse proportion to each other, the clothoid curve CVcl has a shape in which the radius of curvature, R, decreases as the curve length L increases.
When a vehicle is made to travel in such a road section at the same speed, an operation is performed in such a manner that the steering wheel held at a neutral position in the straight section SsIN is turned at the same angular velocity in a clothoid section ScIN, the steering wheel is held at a certain steering angle in the arc section So, and in a clothoid section ScOUT, the steering wheel held at the certain steering angle is turned back to the neutral position by the time the vehicle reaches the straight section SsOUT. By designing a curved section Sc so as to connect the clothoid section Scl, the arc section So, and the clothoid section ScOUT, the vehicle can pass through the curved section Sc smoothly without a sudden steering operation.
There is a road-shape estimating apparatus (corresponding to a curved-section-information supplying apparatus) for estimating the shape of a curved section of a road, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-214839. The road-shape estimating apparatus uses a table indicating relationships between the smallest radius of a curved section (which is assumed to be the radius of an arc section that follows a clothoid section) and clothoid coefficients, to obtain a clothoid coefficient corresponding to the smallest radius of an actual curved section. In accordance with an approximate equation using the clothoid coefficient, the road-shape estimating apparatus determines a clothoid curve (a sequence of a large number of points, for example, 100 points, represented by X and Y coordinates) for the actual curved section. The determined clothoid curve is made to match (fit to) the sequence of node points representing the curved section, and information of the clothoid curve that matches the sequence of the node points is provided as information indicating the shape of the curved section.
In order to determine a clothoid curve, the above-described known apparatus (corresponding to the curved-section-information supplying apparatus) generally needs to calculate coordinate values of a large number of points (e.g., 100 points) representing the clothoid curve, and this calculation processing requires a relatively large amount of time. Also, when providing information of a radius of curvature (or a curvature) at each traveling position in a curved section, the information being useful for speed control, anti-skid control, and so on for a vehicle, the above-described known apparatus further needs to calculate a radius of curvature at each traveling position on the basis of the obtained clothoid curve information.
As described above, the known apparatus requires a relatively large amount of time for calculating a clothoid curve and further involves a calculation for obtaining curved-section information required for various types of control of a vehicle and based on the radius of curvature at each traveling position in a clothoid section. Thus, the known apparatus is not suitable for an application in which it is mounted in a vehicle to supply curved-section information based on the radius of curvature at each traveling position in a clothoid section included in a curved section actually traveled.